Lecture 2-2
Emily Navarro

• Use Cases
– What and why
– How
– Use case diagrams
– Use case models
– Advanced concepts
– Guidelines
• Homework 1
• Quiz #1

• Use Cases
– What and why
– How
– Use case diagrams
– Use case models
– Advanced concepts
– Guidelines
• Homework 1
• Quiz #1

• A depiction of the following requirement information
– Basic functionality
– Any precondition for the functionality
– Flow of events (scenario) for the functionality
– Any postcondition for the functionality
– Any error condition and alternative flow

• A use case is a complete and meaningful flow of events
• A use case captures the contract or
“guarantees” that will hold at the conclusion of the use case

• Use cases are textual descriptions of
– Major functions the system will perform for its users
– Goals the system achieves for its users along the way
• A use case describes a set of flows or scenarios
– Each scenario is a sequence of steps describing an interaction between a “user” and a “system”
– The use case is a collection of scenarios that together accomplish a specific user “goal”
– Each scenario corresponds to a single path or flow through a use case

• A scenario is a sequence of steps describing an interaction between a user and a system
• For an online store, we might use narrative text to describe the following “Buy a Product” scenario*:
– “The customer browses the catalog and adds desired items to the shopping basket. When the customer wishes to pay, the customer describes the shipping and credit card information and confirms the sale.
– The system checks the authorization on the credit card and confirms the sale both immediately as well as with a follow-up email.”
*Adapted from “UML Distilled” by Martin Fowler

• In our Buy a Product scenario things went well…
– It is the “happy day” scenario or Basic Flow
• But things can go wrong…
– You may not need to capture shipping and credit card information for returning customers
• This would be a separate scenario or Alternative Flow
– The credit card authorization might fail
• This is yet another scenario, but an unsuccessful one, or an
Exception Flow
• A use case is a set of scenarios serving a common goal
– The user does not always succeed but the goal remains

• In our Buy a Product scenario things went well…
– It is the “happy day” scenario or Basic Flow
• But things can go wrong…
– The credit card authorization might fail
• This would be a separate scenario or Alternative Flow
– You may not need to capture shipping and credit card information for returning customers
• This is yet another scenario, a second alternative flow
• A use case is a set of scenarios serving a common goal
– The user does not always succeed but the goal remains
A use case should capture all possible scenarios—successful and unsuccessful ones

• Other requirements engineering methods have many limitations
– Do not necessarily map well to design/code
• Require additional work/effort/thought
– Do not translate well to acceptance tests
– Are difficult for non-experts/other stakeholders to understand
• Use cases attempt to bridge the understandability gap
– Describe system behavior, flows of events
– Describe user requests and system responses
– Useful in formulating test steps and verification points

• Capture the intended behavior of the system you are developing
– Without having to specify how that behavior is implemented
• Allow developers to come to a common understanding with your system’s end users and domain experts
• Realized by design and implementation elements working together to carry out each use case
• Help verify and validate your design and implementation features
– Against user goals and requested functions

• Together, the set of use cases specify the complete functionality of the system
– But they do not encompass all of the requirements
• Not good for specifying
– User interfaces
– Data formats
– Business rules
– Non-functional requirements

• Together, the set of use cases specify the complete functionality of the system
– But they do not encompass all of the requirements
• Not good for specifying
– User interfaces
– Data formats
– Business rules
– Non-functional requirements
Produce use cases in conjunction with other requirements engineering methods

– Actors are not part of the system,
• they represent roles a user of the
• system can play user of the system can play
– An actor may actively interchange
• the system information with the system
– An actor may be a provider of
both) or both
– An actor can represent a human, a machine or another system (for example, software, hardware, DB)

– Actors are not part of the system,
• they represent roles a user of the system can play system (e.g., software,
– An actor may actively interchange
• information with the system
• between actors and the system
– An actor may be a provider of information, receive information, or both
– An actor can represent a human, a machine or another system (for example, software, hardware, DB)

• Actors are discovered
– In any documents describing system scope/definition
– By talking with customers and domain experts
• Useful questions for identifying actors include:
– Who uses the system?
– Who installs the system?
– Who starts up the system?
– Who shuts down the system?
– What other devices and external systems work directly with the system?

• Addition questions for identifying actors are:
– Who gets information from this system?
– Who provides information to the system?
– Does anything happen automatically at a preset time?
– Who is interested in a certain requirement?
– Where in the organization is the system used?
– Who will benefit from the use of the system?
– Who will support and maintain the system?
– Does the system use an external resource?
– Does one person play several different roles?
– Do several people play the same role?
– Does the system interact with a legacy system?

• Marketing personnel, human factors engineers, specialty engineers:
Approve what the system should do
• System engineers: Ensure system requirements are met by the use cases
• Reviewers: Examine the flow of events
• Software developers: Use as a basis for analysis, design, implementation
• System and software testers: Use as basis for test cases
• Project leads: Use for project planning
• Technical writers: Use for writing the end user’s guide

• Use Cases
– What and why
– How
– Use case diagrams
– Use case models
– Advanced concepts
– Guidelines
• Homework 1
• Quiz #1

• What functions will the actor want from the system?
• Does the system store information? What actors will create, read, update, or delete that information?
• Does the system need to notify an actor about changes in its internal state?
• Are there any external events the system must know about? What actor informs the system about those events?
• What are the tasks of each actor?
• What use cases will support and maintain the system?
• Can all functional requirements be performed by the use cases?

• Describe only the events needed to accomplish required behavior of the use case
– In terms of what the system should do, not how it does it
– In terms the audience (customer/stakeholder/other) will understand
– Using business domain terminology, not implementation terminology
• The flow of events should describe
– When and how the use case starts and ends
– The interactions (in sequence) between use case and actors
– What data is needed by/exchanged during the use case
– The basic flow (normal sequence) of events for the use case
– Description of any alternative or exceptional flows of events

• Level: Sea Level
• Basic Flow (Main Success Scenario)
1.
Customer browses catalog and selects items to buy
2.
Customer goes to check out
3.
Customer fills in shipping information
4.
System presents full pricing information, including shipping
5.
Customer fills in credit card information
6.
System authorizes purchase
7.
System confirms sale immediately
8.
System sends confirmation email to customer
• Alternative Flow
3a. Customer is regular (repeat) customer
1. System displays current shipping, pricing, and billing information
2. Customer may accept or override defaults, returns to BF at step 6

• Begin by describing the Basic Flow
– Main success scenario
– Sequence of numbered steps
• Add variations
– Alternative Flows
• Still achieve the goal successfully
– Exception Flows
• Fail to achieve the goal

• Each use case has a primary actor
– Has the goal the use case is trying to achieve
– There may be additional, secondary actors
• Each step in the use case flow should be a clear, simple statement
– Show who is engaged and involved in the step
– Show the intent of the actor—what the actor wants, not how the system does it
– Therefore do not describe or include UI details in the text of the use case steps

• Use cases can be as simple as
– a paragraph of informal text
• Or as complex as
– template-based forms that remind developers what information to include
• What to use depends on the formality level of the project
– High formality -> formal templates
– Mid formality -> templates with some of the fields
– Low formality -> paragraphs of text

• Name/title
• Description
• Revision History
• Actors
• System Scope
• Goal
• Level
• Assumptions
• Relationships
– Includes
– Extends
– Extension Points
• Precondition
• Trigger Events

• Basic Flow 1 – Title
– Description (steps), etc.
• Post conditions
• Alternative Flow 1 – Title
– Description (steps)
• Alternative Flow 2 – Title
– Description (steps)
• Alternative Flow 3 – Title
– Description (steps)
• Exception Flow 1 – Title
– Description (steps)
• Activity Diagram
• User Interface
• Special Requirements
– Performance Requirements
– Reports
– Data Requirements
• Outstanding Issues

• Use Cases
– What and why
– How
– Use case diagrams
– Use case models
– Advanced concepts
– Guidelines
• Homework 1
• Quiz #1

• A use case diagram is a graphical view of some or all of the actors, use cases, and their interactions identified for a system
• Each system typically has a Main Use Case diagram
– A picture of the system boundary (actors) and the major functionality provided by the system (use case packages)
• Other use case diagrams may be created as needed
– A diagram showing all the use cases for a selected actor
– A diagram showing all the use cases being implemented in an iteration
– A diagram showing a use case and all of its relationships

System boundary System boundary
ATM User
Withdraw money
Kiosk User
Order tickets
Actor Use Case Actor Use Case

System boundary System boundary
ATM User
Withdraw money
Kiosk User
Order tickets
Actor Use Case Actor Use Case
Use cases are a simple and powerful way to define requirements for software behavior

ATM User
Withdraw Money
Deposit Money
Transfer Money
(Between Accounts)
Bank
System

• Use Cases
– What and why
– How
– Use case diagrams
– Use case models
– Advanced concepts
– Guidelines
• Homework 1
• Quiz #1

• Consists of
– Use cases
• Illustrating the system’s intended functions/behaviors
– Actors
• Illustrating the system’s immediate surroundings
– Diagrams
• Illustrating relationships between the system (use cases) and its surroundings (actors)
• There will generally be one use case model per system, containing multiple use cases, actors, and diagrams

• Used as a unifying thread throughout development
– The same use case model used in requirements is used in design, implementation, and test
• Used to identify
– Who will interact with the system and what the system should do
– What interfaces the system should have
• Other related requirements documents may be linked

• Used to communicate with the end users and domain experts
– Provides buy-in at an early stage of development
– Ensures a mutual understanding of the requirements
• Used to verify that
– All behavioral (system interaction) requirements have been captured
– Developers have understood the requirements

• Used to communicate with the end users and domain experts
– Provides buy-in at an early stage of development
– Ensures a mutual understanding of the requirements
• Used to verify that
– All behavioral (system interaction) requirements have been captured
– Developers have understood the requirements
The most important role of a use case model is to communicate the system’s functionality and behavior to the customer or end user

• Use Cases
– What and why
– How
– Use case diagrams
– Use case models
– Advanced concepts
– Guidelines
• Homework 1
• Quiz #1

• Levels of granularity
• Use case “includes”
• Use case “extends”

• Cockburn’s use case level of granularity or “level” is useful but challenging
– Core use cases are at “sea level”
• An interaction with an actor toward a visible tangible goal
• Can be done at one sitting at a computer
• Withdraw money, deposit a check, check balance
– Fish level
• Use cases that are included by sea-level use cases
• Authorize customer, generate receipt
– Kite level
• Show how sea-level use cases fit into larger business context
• Also called summary-level or business-level use cases
• Cannot be done in one sitting, and may require multiple people, organizations, and systems interacting
• Manage ATM operations, manage investments, manage loans

• One use case includes another use case in its entirety
– Analogous to a program calling another or a routine using a subroutine
– The “call” is mandatory
• The calling/including use case must flow through the included use case
– An application of reuse, modularity, anticipation of change
• Multiple use cases share the same functionality
• This functionality is placed in a separate use case
• Avoids repetition of the same information in multiple use cases
– Examples
• Logon/logoff
• User authentication/authorization

• An extends relationship is used to show
– Optional behavior
– Behavior that is only run under certain conditions, such as triggering an alarm
– An “interruption” in the basic flow when the condition comes true
• For example, a use case that monitors the flow of packages on a conveyer belt can be extended by a
Trigger Alarm use case if the packages jam
• Typically occurs when an alternative flow has gotten too big for a particular use case

• Use Cases
– What and why
– How
– Use case diagrams
– Use case models
– Advanced concepts
– Guidelines
• Homework 1
• Quiz #1

• If you don’t fully understand the ins and outs of use cases, it is easy to misuse them or turn them into “abuse” cases
• Ellen Gottesdiener: “Top Ten Ways Project Teams Misuse
Use Cases – and How to Correct Them.” The Rational Edge,
June 2002 (Part I), July 2002 (Part II).
• Martin Fowler: “Use and Abuse Cases.” Distributed
Computing, April 1998.
• Doug Rosenberg: “Top Ten Use Case Mistakes.” Software
Development, February 2001.
• Susan Lilly: “How to Avoid Use Case Pitfalls.” Software
Development, January 2000.
• Kulak and Guiney: “Use Cases: Requirements in Context.”
Second Edition, Addison-Wesley 2003.

• Don’t bother with any other requirements representations
– Use cases are the only requirements model you’ll need!
• Stump readers about the goal of your use case
– Name use cases obtusely using vague verbs such as “do” or
“process”
• Include nonfunctional requirements and UI details in your use case text
• Use lots of extends and includes in your initial use case diagrams
– This allows you to decompose use cases into tiny units of work

• Don’t involve subject matter experts in creating, reviewing, or verifying use cases
– They’ll only raise questions!
• If you involve users in use cases definitions at all, just
“do it”
– Why bother to prepare for meetings with the users?
• Write your first and only use case draft in excruciating detail
– Why bother iterating with end users when they don’t even know what they want?
• Don’t validate or verify your use cases
– That will only cause you to make revisions and do more rework!

• Rigor and formality
• Separation of concerns
– Modularity
– Abstraction
• Anticipation of change
• Generality
• Incrementality
These principles apply to all aspects of software engineering

• Use Cases
– What and why
– How
– Use case diagrams
– Use case models
– Advanced concepts
– Guidelines
• Homework 1
• Quiz #1

• You will specify the requirements for
ZotMyHealth
– ZotMyHealth will allow users them to keep track of their personal health data (sleep patterns, workouts, calorie intake, etc.)
• Homework 1 has been posted
– Along with a template, rubric, and some example requirements documents
– All of the instructions are contained in the prompt and in the template

• There is no template to follow for the use case part, but see samples for acceptable examples

• Client interview will span Tuesday, April 14,
Thursday, April 16, and Tuesday, April 21
• Read the prompt and template
• Come prepared with your questions
• Research existing similar systems
• For this exercise, Anirudh is a “naïve” client who
– knows very little about software engineering
– knows only about his business
– will not answer any questions outside of lecture

• Client interview
• Discussion tomorrow

• Use Cases
– What and why
– How
– Use case diagrams
– Use case models
– Advanced concepts
– Guidelines
• Homework 1
• Quiz #1